Solid polymer fuel cells using alcohol fuel are easy to reduce in size and weight. For this reason, they are now actively researched and developed as power supply for various electronic devices comprising portable devices.
Solid polymer fuel cells comprise a membrane-and-electrode assembly (MEA) in which a solid polymer electrolyte membrane is held between a fuel electrode and an oxygen electrode. The fuel cells of a type designed to supply liquid fuel directly to the fuel electrode are referred to as direct fuel cells, and produce cations, electrons, and intermediates by decomposing the supplied liquid fuel with catalyst supported by the fuel electrode. In this type of fuel cells, the so produced cations move to the oxygen electrode side, passing through the solid polymer electrolyte membrane, and the so produced electrons move to the oxygen electrode side via an external load, so that the cations and the electrodes react with oxygen in air in the oxygen electrode to generate power. For example, in the case of a direct-methanol-type fuel cell (hereafter, referred to as the DMFC) using aqueous methanol solution directly as liquid fuel, a reaction represented by the following formula 1 takes place in the fuel electrode, while a reaction represented by the following formula 2 takes place in the oxygen electrode.CH3OH+H2O→CO2+6H++6e−  16H++6e−+3/2O2→3H2O  2
In order to ensure stable operation of the fuel cell using the reaction formulae above, it is necessary to constantly supply the MEA fuel electrode functioning as a power generation portion with alcohol fuel such as methanol in an amount required for power generation. Conventionally, means for supplying alcohol fuel to an MEA serving as a power generation portion has been widely been provided by a method in which a tank storing liquid fuel is incorporated in a fuel cell body or externally attached thereto in the form of a cartridge for supplying the liquid fuel directly.
Japanese Laid-Open Patent Publication No. 2004-71262 (Patent Document 1), for example, discloses a fuel cell designed to feed liquid fuel from a fuel cartridge to a fuel storage layer integrated with a power generation portion by means of a liquid feeding pump, for supplying the fuel to a fuel electrode of a MEA as the power generation portion.
Japanese Laid-Open Patent Publication No. 2006-49032 (Patent Document 2) discloses a fuel cell system in which a collecting tank is provided for collecting product water and by-products produced in an electrostatic generator.
Japanese Kohyo Patent Publication No. 2005-524952 (Patent Document 3) proposes a technique to separate a power generation section from a fuel cartridge section by using a detachable cartridge comprising a perforated porous structure, claiming that the fuel supply is possible with a simple configuration without the need of a collecting mechanism as described above.
Japanese Laid-Open Patent Publication No. 2006-19162 (Patent Document 4) discloses a technique in which a plurality of gas diffusion layers comprising regions with different gas permeability are stacked, and the gas permeability of the gas diffusion layers is changed by changing their overlapping condition.
Japanese Kohyo Patent Publication No. 2004-529460 (Patent Document 5) discloses a fuel cell for obtaining high efficiency in a low load region by adjusting the size of the cell surface where fuel cell reactions occur by moving a porous plate in a path for supplying a reaction medium.
Japanese Laid-Open Patent Publication No. 2006-269126 (Patent Document 6) discloses a fuel cell in which three membranes comprising a hydrophobic porous membrane, a polymeric swollen membrane, and a gas-liquid separation membrane are stacked in this order from the fuel electrode side for realizing uniform supply of fuel.